1. Field of the Invention
The present invention relates to a composite member, its separation method and a preparation method of a semiconductor substrate, particularly to a composite member having inside a fragile structure with a low mechanical strength, its separation method and a preparation method of a semiconductor substrate. The present invention is particularly suitable for a preparation method of a substrate (SOI substrate) having an SOI (semiconductor on insulator) structure as a type of semiconductor substrate.
2. Related Background Art
A device using the SOI substrate has various advantages which cannot be attained by an ordinary Si substrate. For example, the advantages are as follows:
(1) a dielectric is easily separated, and the device is suitable for high integration;
(2) the device is superior in resistance to rays;
(3) a floating capacity is small, and high speed operation of elements is realized;
(4) a well process is unnecessary;
(5) latch-up can be prevented; and
(6) a complete depletion type field-effect transistor can be formed by thinning films.
Since the SOI structure has various advantages as described above, researches concerning its forming method have been advanced these several decades. Known as a conventional SOI technique is an SOS (silicon on sapphire) technique for forming Si on a single-crystal sapphire substrate by hetero epitaxial growth in CVD (chemical vapor deposition) method. The SOS technique has been evaluated as most matured SOI technique, but has not been put to practical use because of a large amount of crystal defects caused by lattice mismatching in an interface of an SI layer and a base sapphire substrate, mixture of aluminum constituting the sapphire substrate into the Si layer, substrate price, delay in area enlargement, and for other reasons.
Following the SOS technique, an SIMOX (separation by ion implanted oxygen) technique has appeared. Concerning the SIMOX technique, various methods have been developed aiming at reduction of the crystal defects or reduction of manufacture cost. Examples of the methods include a method of injecting oxygen ions to a substrate to form an embedded oxide layer; a method of bonding two wafers via an oxide film, polishing or etching one of the wafers, and leaving a thin single-crystal Si layer on the oxide film; a method of implanting hydrogen ions into a predetermined depth from a surface of an Si substrate with an oxide film formed thereon, bonding with the other substrate, leaving a thin single-crystal Si layer on the oxide film by heating or another treatment, and peeling off the bonded substrate (other substrate); and the like.
A new SOI technique has been disclosed in Japanese Patent No. 2,608,351 or U.S. Pat. No. 5,371,037. In the technique, a first substrate obtained by forming a non-porous single-crystal layer on a single-crystal semiconductor substrate with a porous layer formed thereon is bonded on a second substrate and these substrates are bonded, then unnecessary portions are removed, so that the non-porous single-crystal layer is transferred to the second substrate. The technique is superior in that the SOI layer has a superior film thickness uniformity, a crystal defect density of the SOI layer can be reduced, the SOI layer has a good surface flatness, a manufacture device with expensive and special specifications is unnecessary and that SOI substrates having SOI films in the range of about several 10 nm to 10 xcexcm can be manufactured with the same manufacture device.
Furthermore, the present applicant has disclosed a technique in Japanese Patent Application Laid-open No. 7-302889, in which after first and second substrates are bonded, the first substrate is separated from the second substrate without being collapsed, a surface of the separated first substrate is then smoothed, and a porous layer is formed thereon again, so that the first substrate is reused. An example of the proposed method will be described with reference to FIGS. 12A to 12C. After a surface layer of a first Si substrate 1001 is made porous to form a porous layer 1002, a single-crystal Si layer 1003 is formed on the layer 1002, and the single-crystal Si layer and a main surface of a second Si substrate 1004 separate from a first Si base substrate are bonded to each other via an insulating layer 1005 (FIG. 12A). Thereafter, a wafer bonded via the porous layer is divided (FIG. 12B), and the porous Si layer exposed to the surface of a second Si base substrate is selectively removed to form an SOI substrate (FIG. 12C). The first Si substrate 1001 can be reused by removing the remaining porous layer therefrom.
In the invention disclosed in Japanese Patent Application Laid-open No. 7-302889, the substrate is separated using the property that the structure of the porous silicon layer is more fragile than a non-porous silicon. Since the substrate once used in preparation of the semiconductor substrate can be used again in preparation of the semiconductor substrate, the cost of the semiconductor substrate can effectively be reduced. Moreover, in the technique, since the first substrate can be used without being wasted, the manufacture cost can largely be reduced. Additionally, the manufacture process is advantageously simple.
Examples of the method for separating the first and second base substrates (base plates) include pressurizing, pulling, shearing, wedge insertion, thermal treatment, oxidization, vibration application, wire cutting, and the like. Additionally, the present inventors have proposed a separation method in Japanese Patent Application No. 9-75498 or U.S. patent application Ser. No. 047,327 filed on Mar. 25, 1998, in which fluid is sprayed to a separation area. Gas and/or liquid is used as the fluid, and especially a water jet using a liquid mainly composed of water is preferable. In the method, at the time of separating, water not only cuts a bonded face but also uniformly enters a gap-between the first and second bases, so that a relatively uniform separating pressure can be applied to the entire separation face. Moreover, in the method, different from the case where gas is not used, particles can be washed away without being scattered. The method is superior in these two respects to the separation method by the wedge insertion. Especially, when the mechanical strength of the separation area is set lower than that of the bonded place, only the fragile portion is ruptured, collapsed or removed by spraying the fluid flow to the separation area, and another strong portion can advantageously be left without being collapsed.
However, when the water jet or another fluid is used to separate the bonded composite member by spraying the fluid to a side face of a composite member, especially around a side face of the separation area, there is a case where the fluid flow cannot easily break or cut the separation area because the separation area has an excessive strength. In this case, the composite member can be separated by raising a fluid pressure, but if the pressure is excessively raised, cracks advance inward from the side face of the bonded base. In the midst, one or both of the separated base substrates may be cracked by the pressure of the fluid injected to the separation area. Therefore, yield is lowered in the separating process. To avoid this, there is provided a method of further lowering the mechanical strength of the separation area to form a more fragile structure. If the structure is excessively fragile, however, a problem is caused that the separation area is broken and cannot be bonded or that the separation area is broken to generate particles as contaminants during heating, cleaning, or handling the base substrate otherwise in a process of preparing the composite member.
Moreover, when separation is performed in another method without using the fluid, basically the similar problem is caused. Therefore, the yield in the separating process may be lowered.
An object of the present invention is to provide a composite member and its separation method in which the composite member can be separated relatively easily without damaging separated bases.
Another object of the present invention is to provide a composite member and its separation method in which a mechanical strength of a major separation area can be relatively raised, unintended collapse of the separation area is prevented, and generation of particles is suppressed.
According to an aspect of the present invention, there is provided a preparation method of a semiconductor substrate using a separation method of a composite member comprising the step of separating the composite member into a plurality of members at a separation area, in which a mechanical strength of the separation area is non-uniform along a bonded face.
Especially, in the separation area, a peripheral portion of the composite member is preferably lower in mechanical strength than a central portion. Additionally, the separation area is preferably lower in mechanical strength than the bonded interface.
According to another aspect of the present invention, there is provided a preparation method of a semiconductor substrate using the separation method mentioned above.
According to still another aspect of the present invention, there is provided a preparation method of a semiconductor substrate which comprises separating a composite member formed by bonding a first base substrate and a second base substrate to each other into a plurality of members at a separation area formed in a position different from a bonded face, a mechanical strength of the separation area being non-uniform along the bonded face, and a mechanical strength of a peripheral portion of the separation area being locally low.
According to a further aspect of the present invention, there is provided a composite member comprising a separation area inside, a mechanical strength of the separation area being non-uniform along a surface of the composite member, a mechanical strength of a peripheral portion of the separation area being locally low.
For the separation area, a porous layer formed by anodization, a layer formed by implanting ions in which microcavities can be obtained, or the like can be used. When an Si wafer or another semiconductor substrate, or a quartz wafer is used as a first or second base, it substantially has a disc shape although it has an orientation flat or a notch. Therefore, the composite member obtained by bonding the first and second base substrates to each other also has a substantially disc shape. In this case, when the mechanical strength of the separation area is non-uniform in such a manner that the strength is high in the central portion of the composite member and low in its peripheral portion, and substantially uniform in a circumferential direction, the composite member can effectively be separated. When the composite member is a rectangular plate member, the mechanical strength of its corner, side or entire periphery is lowered.
The mechanical strength can be made non-uniform by forming portions different in porosity from one another in the separation area. As the porosity is increased, the mechanical strength is lowered. Therefore, the mechanical strength can be changed by changing the porosity. Specifically, the mechanical strength of the peripheral portion can be lowered by setting higher the porosity in the peripheral portion than in the central portion.
The mechanical strength can also be made non-uniform by changing a thickness of the separation area. As the thickness of the separation area is increased, the mechanical strength is lowered. Therefore, the mechanical strength is also changed by changing the thickness. Specifically, the mechanical strength of the peripheral portion can be lowered by setting a thickness of a porous layer of the separation area larger in the peripheral portion than in the central portion of the base.
In order to obtain a suitable composite member which fails to be separated in a process prior to a process of separating the composite member and is securely separated in the separation process, the separation area is preferably formed by a plurality of layers different in mechanical strength. Especially, in the separation area comprising the plurality of layers, the thickness of a layer high in porosity is preferably less than the thickness of a layer low in porosity adjacent to a non-porous single-crystal semiconductor layer. A structure of each of the plurality of layers does not necessarily have to be steeply changed in an interface. Even if the strength or structure of each layer is continuously changed in the interface of the adjacent layers, separation is facilitated as compared with when the strength is uniform over the entire separation area.
In the separation area comprising the plurality of layers different in the mechanical strength, the layer high in porosity preferably has a higher porosity in the peripheral portion than in the vicinity of the central portion of the base.
In the separation area comprising the plurality of layers different in the mechanical strength, the porosity of a second layer with a high porosity can be made higher in the peripheral portion than in the central portion of the base substrate by making the thickness of a first layer with a small porosity larger in the peripheral portion than in the central portion of the base.
The present inventors have conducted experiments in which an anodization device is variously modified to form a good-quality porous layer. As a result, they have found that there is an Si wafer having an in-plane porosity distribution among a plurality of Si wafers subjected to a porous treatment using a certain mode of anodization device. Moreover, as a result of experiments in which samples are prepared by forming non-porous layers on porous layers and the non-porous layers are peeled off, it has been found that in some of the samples even the porous layer relatively low in porosity can be peeled off more easily than the layer relatively high in porosity. It is seen from the aforementioned two findings that, as in an embodiment described later, when the layer relatively high in porosity is ruptured or collapsed in the porous layers having the in-plane distribution of porosity, the layer relatively low in porosity is also easily ruptured, which is not much influenced by an absolute value of porosity.
Specifically, it has been found that when there is a layer relatively high in porosity in the peripheral portion of a member in which separation can easily be started, the separation is facilitated regardless of the absolute value of porosity, and the present invention has been developed.